Centrifuge



Feb. 1, 1944. K G. STERN CENTRIFUGE Filed sept. 9, 1941,-

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GENTRIFUGE Filed Sept. 9, 1941 4 Sheets-Sheet 2 o 3 I MN Q l a@ mw MM Wa o w M 00H .ww .Il vvU-UUUUU :1;: e

Feb. l, 1944. 4

K. G. STERN CENTRIFUGE Filed Sept. 9, 1941 4 Sheeis-Sheet 3 WWII/I4latente'd Feb. l, 1944 CENTBIFUGE Kurt G. Stern, New Haven, Conn.,assigner to Research Corporation, New York, N. Y., a corporation of NewYork Application September 9, 1941, Serial No. 410,178

8 Claims.-

The present invention relates to improvements in centriiuges, and isadapted 'for use as an analytical ultra-centrifuge. and for the opticalstudy in intense gravitational ilelds during high-speed centrifugation,of the boundaries of molecularlydispersed colloids of organic andinorganic nature as well as the behavior of microscopical objects suchior example as single cells.

One object oi the present invention is to provide an improved centrifugeof simple construction.

Another object of the present invention is to provide an improvedcentrifuge capable of accomplishing results not possible with previouscentrifuges.

With the above and other objects in view, as will appear to thoseskilled in the art from the present disclosure, this invention includesall features of the4 said disclosure which are novel over the prior art.

Owing to the enormous stresses and consequent strains which occur incentriiuges, high-strength materials such as steels and light-metalalloys have heretofore been employed for the largerdiametered rotatingportions ci centrii'uges of the nature herein dealt with. 'Ihe inventionbroadly comprises the construction of the main, stresscarrying rotatingmembers of centrifuges of plastic compositions. Centrifuges constructedof such compositions in accordance with my invention worked with perfectsatisfaction without bursting under the stresses set up in the rotatingparts, and resulted in advantages and new results which are entirelyimpossible of attainment with rotors formed of metal. While I do notrely upon any particular theory as to why Centrifuges in which therotors are formed of plastic are not only successful but give unexpectednew results, it appears that part of the explanation resides in the factthat the plastic vhas a very low modulus of elasticity as compared toeither steel or aluminum alloys or other metals.

Where such terms as "plastic, plastic resins, synthetic resins or"plastic material are used, it is intended to refer to such types ofmaterials for example as polyacryllc, polystyryl, polyvinyl, phenolformaldehyde, cellulose nitrate and cellulose acetate plastics. Owinglargely to the optical advantages oi polymerized methyl methacrylateresin, commercially known as Luclte, whereby this material is obtainablein very high transparency free from any color, I have preferred toconstruct my centrifuges oi this material. Luclte has a tensile strengthof from 9,000 to 12,000 lbs. per square inch, a speciiic gravity oi from1.18 to L20. and a modulus of elasticity o! `approximately between500,000 and 1,000,000 as compared with a modulus oi elasticity of30,000,000 for steel.

It will thus be seen that the modulus of elasticity ot Luclte is onlyfrom 1/an to 1/eu oi the modulus of elasticity of steel, which meansthat Luclte will stretch within its elastic limit from thirty to sixtytimes as i'ar. as steel will stretch under the same stress per unit ofarea. Thus, it will be seen that in a rotor constructed of Lucite. eventhough there be certain irregularities of internal stresses in the rotorwhen at rest, under the high stresses resulting from centrifugal forcethe Luclte tends, owing to the very low modulus of elasticity of thismaterial, to avoid producing a higher localized stress at any locationthan the material is capable oi standing, thus minimizing the tendencyof a stress at any location in a Luclte rotor from rising higher thanthe material can stand, thus avoiding starting a rupture at such alocation. It will be appreciated, that if in any rotating rotor, a unitor liber stress is induced which is greater than the tensile strength oithe material at that point, rupture will be started there and ordinarilywill rapidly extend through adjacent portions of the rotating rotor toquickly result in bursting of the rotor. But whatever the correctexplanation may be as to the fact that rotors made of plastic,surprisingly withstand the stresses involved, the fact remains thatunexpected results and advantages ow from the employment oi rotors madeof plastics.

Another important advantage resulting from rotors made of plastic. isthe low specific gravity oi plastic. Thus, Luclte has a specific gravityor density as above given, which approaches closely the speciiic gravityor density of the' solutions to be studied in these centrifuges.Heretofore in the making of rotors of metal, it has been necesl sary tomake an insertable and removable anain the cell to be examined, is ofsubstantially the.

same `speciiic gravity as the rotor itself, it is unnecessary to placeany balancing cell in the rotor,

' of the rotor.

thus providinga stronger simpler construction.

And even greater strength is obtained in the rotor when the analyticalcell forms an integral part Where I use the term ultracentrifuge," Imean a device which permits of molecular sedimentation undisturbed bymechanical vibrations or thermal convection currents induced in thedispersed fluid system under study which may, for example, be a smoke, asolution or a mechanical suspension, the definition of anultracentrifugenot necessarily being linked with the intensity of the centrifugal fieldemployed.

Centrifuge rotors made in accordance with my invention run substantiallyfree from mechanical vibration, even when the rotor is horizontally Amounted, and in a commercial type of roller bearing. This important factis due to the low specific gravity of plastic, whereby the total weightof the rotor is so light that only a small load is placed'on thebearings, so that it has been possible to employ a commercial type ofroller bearing instead of the elaborate and costly pistonactuatedjournal bearings employed in the oildriven steel-rotor ultracentrifugesof Svedberg.

In the case of centrifuge rotors of high heatconductivity, such as metalrotors, a considerable part of the cause of thermal convection currentsinduced in a dispersed fluid system in the cell of the rotor, is due tothe friction of the axle of the rotor with the bearings, and thediffusion of the heat thus generated, from the center of rotation tonear the periphery of the rotor where the iluid system under study islocated. Rotors in accordance with my invention, made of plastic. on theother hand, are characterized by their relatively-low heat conductivity,which property leads to the establishment of a barrier between thesource of heat-production, that is, the axle, and the cell containingthe fluid material under study, thus substantially preventing heat fromthe axle being transmitted to and heating the fluid-containing cell, andthus preventing` convection currents arising from this heat.

The fact that the plastic material has a low density which approachesthat of the solution to be placed in the cell-chamber to be centrifugedand that both the rotor which carries the cell and the cell itself canboth be made of the same density of material, results in substantialhomogeneity of the field during centrifuging and limits the stressesarising during spinning to the minimum possible.

The low density of the plastic, the fairly-high tensile strength of theplastic and its relativelylow modulus of elasticity permit of obtainingthe ideal thin cylindrical disk-shape of a high-speed rotor. Thus, theratio between thickness at the cell-holes and diameter is from 0.22 to0.27 in shell rotors, whereas in the case of a 6" plastic rotor such asshown in Figs. 1 to 3, this ratio has been reduced to 0.066.

Rotors either for centrifuging use or for test purposes can be made oftransparent plastic, and by viewing the'rotors either when at restagainst a diffused light source with the rotors located between twocrossed Polaroid screens or Nicol prisms will show stress patterns inaccordance with any unequal stresses introduced by machining thecell-retaining holes in the rotor and machining the arborandscrew-receiving holes near the center of the rotor, etc. Stressesresulting from machining and from fastening the arbor in the rotor, canbe removed by an annealing process consisting in subjecting the rotor tocontrolled heating to the softening point and slowly subsequentlycooling the rotor, employing observations through polarizing optics as ameans ci accurate control. Also, stress patterns introduced in rotorsduring rotation of the rotors at high speed can be studied by employingpolarizing optics in connection with the well-known type of stroboscopiclighting device.

As the detailed description of the particular forms of the inventionshown in the drawings are explained, the foregoing advantages togetherwith a number of additional advantages will become clear.

In the accompanying drawings forming part of the present disclosure, inwhich certain ways of carrying out'the'lnvention are shown forillustrative purposes:

Fig. 1 is a longitudinalvertical sectional viewy partly in elevation,through a centrifuge made in accordance with the present invention;

Fig. 2 is a vertical sectional view on the plane indicated by the line 22 of Fig. l;

Fig. 3 is a perspective view of the rotor of the centrifuge shown inFigs. 1 and 2;

Fig. 4 is an enlarged front elevation, partly in section, of theremovable analytical cell shown removably mounted in the rotor in Figs.1 and 2:

Fig. 5 is a vertical sectional view on the plane indicated by line 5-5of Fig. 4;

Fig. 6 is a sectional view similar to Fig. 5 of an analytical cell inwhich the thickness of the parts is of different proportion;

Fig. 7 is a front elevation, partly in section, of a modified form ofrotor in which the analytical cell is built as an integral portion ofthe rotor;

Fig. 8 is a vertical sectional view, partly in elevation, on a planeindicated by line 8 8 of Fig. '1:

Fig. 9 is a view similar to Fig. 8 of a rotor in which the thickness ofthe plastic parts is of different proportion;

Fig. 10 is a fragmental sectional view similar to the upper portion ofFig; 8, of a modified form of rotor construction;

Fig. 11 is a fragmental sectional view similar to the upper portion ofFig. 8, of another modified form of rotor construction;

Fig. 12 is a front elevation similar to Fig. 7, of a rotor having aplurality of cells similar to the cell shown in Fig. l1; y

Fig. 13 is a front elevation similar to Fig. 12` of a rotor having aplurality of cells located at different radii from the center of therotor;

Fig. 14 is a front elevation of a modified form of rotor withair-nozzles shown in driving position with relation to the rotor;

Fig. 15 is a left-hand elevation of the construction shown in Fig. 12with the upper or cell portion thereof shown in section;

Fig. 16 is a view similar to Fig. 1'5 of another modified form of rotoremployed in connection with a vacuum chamber indicated diagrammaticallyin broken-line outline;

Fig. 17 is a view similar to Fig. 16 of still another modified form ofrotor employed in connection with a vacuum chamber indicateddiagrammatically in broken-line outline; and

Fig. 18 is a view similar to Fig. l'l of a modified form of rotor forrotation about a vertical axis in a vacuum chamber indicateddiagrammatically in broken-line outline.

Referring to Figs. 1 to 5inclusive which illustrate one form ofcentrifuge made in accordance with the present invention, a base 20supports a housing formed as two parts or members 2l and 22, the lowerhousing part or member 2 serving as a support to carry the bearings 23Afor the rotor 24, and preferably being secured to the base 29 by anysuitable means such for example as angle-members 25 and screws 25.

In an actual embodiment of the invention which was constructed inaccordance with the showing in Figs. 1 to 5 inclusive, the rotor 24 wasformed of 'a single sheet or plate of cast Lucite having an outsidediameter of 6", and having its periphery 21 of general circular form andprovided with utings or blades 23 milled therein to drive the rotor 24as a turbine by means of a stream of air under pressure. The rotor 24has a round central hole 23 in which is press-fitted a brass bushing 30.A plurality of screws 3| extend through holes in the flange 32 of thebushing 30 and threadedly engage in a brass clamping-ring 33. Ifdesired, the threaded ends of the screws 3| can be upset to inhibit anypossible tendency of them to become loose. A shaft or arbor 34 has aforce or press tlt in the bushing 30 and is formed or toolor other steelwhich can be hardened. The opposite ends of trunnions 35 of the arbor 34are about PAB in diameter and hardened and polished so as to form theinner raceof the e" roller bearing 23 known as a Torrington needlebearing, the outer race 35 of which has a force or press ilt in theside-plates 37 of the housing part 2|.

The rotor 24 is provided with a cylindrical hole 38 therethrough toprovide a cell-retaining chamber for an analytical cell 39'which latteris made to have a suitable friction fit within the hole 33 so that thecell 39 can be pushed into place in, or pushed out of place out of, thehole 33, by moderate pressure by a nger.

The analytical cell 39 has a cell-chamber 40 of sector-shape. The cell39 is formed from three sheets or plates of Lucite 4|, 42 and 43integrally united together in any suitable way as for ex-A ample byemploying a Lucite cement or any suitable solvent of Lucite. As Lucitecement polymerizes to form Lucite, the entire assembly of sheets ordisks therefore becomes one indissoluble unit of Lucite throughout. Thetwo outside sheets or plates or Lucite 4| and 43 which overlie oppositesides of the cell-chamber 40', are preferably formed of transparentcolorless Lucite, while the intermediate or spacer sheet or plate ofLucite 42 is preferably formed of opaque Lucite, although it may beformed of a colored transparent Lucite, the object being to have theintermediate sheet 42 of relatively-less light-transmitting power thanthe outside sheets 4| and 42, so as to aid in optically dening thecellchamber and aiding in clear optical and photographical denition ofthe boundary of the cellchamber when the device is in use duringrotation. A small hole 44 is provided so that the liquid to becentrifuged can be inserted into and removed from the cell-chamber 40 bymeans of v a hypodermic syringe.

In orderto supply compressed air or other 'at 41, and has the other endshaped as shown to form a nozzle 43 to supply the air in a streamagainst the utings or blades 4l of the rotor 24 for the purpose ofdriving the rotor at high speed.

Openings 43a through the opposite walls 31 of the housing-part 2|provide a ready escape for the air which issues from the nozzle 4l.

Theprotective cover or housing-member 22 is preferably formed of iron orsteel of sumcient thickness and strength to provide a safeguard againstdanger in event of bursting of thev rotor. The housing-member 22 may beprovided with angle-members 43 connected thereto by screws 5U, and mayif desired be connected to the base 2li by its screws 5|. Thehousing-member 22 is provided with holes 52 through its side-walls inline with each other and in line with the hole 33 of the rotor at oneposition of rotation of the rotor, such for example as itsvertically-uppermost position. The vertical slot 53 in thehousing-member 22 permits it to clear the tube 45 when thehousing-member 22 is placed in its position as shown or is removed fromsuch position, and the horizontal slots or recesses`54 are to providefor ready escape of the air which issues froml the openings 43a in thehousing-member 2|.

Employing well-known apparatus which passes intermittent ashes of lightat a proper flashing rate to synchronizewith the speed of rotation. ofthe analytical cell and having the flashes occur when the cell is in itsvertically-uppermost position properly aligned with the holes 52 throughthe opposite walls of the housing-member 22, from one side of the'housing, permits of the use of any suitable optical mechanism such as amicroscope for optically observing the sedimen-v tation of the materiallocated in the cell-chamber 40 of the cell 39. Instead of employing amicroscope for optical observation by an observer,

-a suitable photographic mechanism can be employed for photographingsedimenting boundaries resulting from the sedimentation induced by thecentrifugal force resulting from rotation ofthe rotor.

Fig.- 6 illustrates an analytical cell 33a similar to the analyticalcell 39 shown in Figs. 4 and 5 except .that the proportions of thethickness of the layers of plastic used in building up the cell aredifferent. Thus, the cell 39a has a thinner cell-chamber 40a formed by arelatively-thin opaque or colored intermittent or spacer sheet ofplastic 42a cemented or otherwise integrally united with colorlesstransparent sheets of plastic 4|a and 43a. Thus, by having theintermediate opaque or colored lamination of any thickness or thinnessdesired. and by having the transparent colorless observation-window 4|aof any thinness practical or thickness desired, the distance of theanalytical cell 40a from the outside surface of the observation-window4|a may be varied at will to a rotor 55 which is similar to the rotor 24shown in Figs. 1 to 3, except that this rotor is formed of three sheetsor laminations of plastic cemented together instead of being formed of asingle sheet of plastic, thus permitting of providing the rotor 55 withan analytical cell 55 which is formed as an integral part of the rotor55 to provide a cellchamber l51.V 'lhis integral cell 55 is formed asslot or space to form the cell-chamber l1 when the spacer-sheet 58 iscemented between two outside or window sheets or laminations 68 and 8U.In this form of the invention an angularlydirected small hole 8i isdrilled through one of the sheets 59 or 6l! into the bottom portion ofthe cell-chamber 5l to provide for illling the cellchamber with thematerial to be centrifuged and to remove it therefrom by means of ahypodermic syringe or the like.

The form of rotor 55a illustrated in Fig. 9 is substantially the same asthat illustrated in Fig. 8 except that the proportions or thicknesses ofthe cell-chamber Sla and or the' sheets 58a, 59a and 60a of plastic areci similar proportions respectively to the cell-chamber 40a and sheets42a, la and 43a of plastic of the analytical cell 39a shown in Fig. t.

Fig. 10 shows an enlarged fragmentai portion i of a rotor 62 andintegral cell similar to the construction shown in Figs. 'l and 8. Inthe construction shown in Fig. 1d the cell-chamber 63 may be formed bymachining it in a relativelythick sheet of plastic 6G and polishing itsbottoni. A transparent colorless sheet of plastic 65 is cemented to thesheet 6G. A hole 66 can be drilled either from the inside before the twosheets are cemented together or can be drilled from the outside afterthe two sheets have been cemented together. If desired, the portion ofthe surface 6l of sheet 6H surrounding and adjacent the cellchamber 62can be coated with opaque or colored material prior to or at the time ofcementing the two sheets together. Or an opaque or colored. cement canbe employed for cernenting the two sheets together.

The form oi construction illustrated in Fig. ll illustrates a rotor 68formed of a single sheet or plate of transparent plastic which has ananalytical cell-chamber B9 drilled therein from the outer periphery ofthe rotor, a iiat seat 1d being formed to receive thereagainst a washerli ci' soft rubber, suitable plastic or other suitable material, whichis pressed into liquid-tight position by means of a screw l2 of plasticwhich screwthreadedly engages a correspondingly screwthreaded hole inthe rotor. The material to be centrifuged in the cell-chamber t@ isinserted therein before the packing li and screw 'l2 are inserted inplace, of course, and in order to remove the centrifuged materialtherefrom, the packing lli and screw l are removed.

The outer generally-circular periphery of all y of the forms of rotorheretofore illustrated are provided with fiutings or blades whereby therotors can be driven by compressed air or the like, as has been morefully described concerning the rotor 2t shown in Figs. l to 3.

The form or" construction illustrated in Fig. 12 illustrates a rotor 'i3formed of transparent plastic which has a plurality of analytical cellshaving cell-chambers 16 drilled therein from the outer periphery of therotor, a fiat seat 'l5 being formed to receive thereagainst a washer 1which is pressed into liquid-tight position by means or a screw 'Il ofplastic which screW-threadedly engages a correspondingly screw-threadedhole in the rotor, the construction of each of these analytical cellsbeing similar to the analytical cell illustrated in Fig. ll. The tubularcell-chambers 'ld are preferably of small diameter corresponding tohematocrit tubes used in the determination of cell-volumina for clinicaland research purposes. By placing the fluid to be examined. for example,blood. in the cell-chambers 14 and properly sealing the cell-chamberswith the washers 'I8 and screws 11, a number of samples of fluid can becentrifuged at the same time by rotation of the rotor 18. If a beam ofparallel light or of divergent light is directed transversely throughthe transparent rotor onto a graduated screen, the progress oi'sedimentation and packing process during centrifuging, can be observedon the screen. In this way, centrifuging can be continued until theimage of the tubes, as projected on the screen, no longer shows anycontraction of the blood-cells in the analytical cells. If severalanalytical cells of the same length and radial location are filled withthe same iuid, an optical average of the blood-cell volume can besecured in this way by observing the composite image of all of theanalytical cells or tubes contained in the rotor. For individualmeasurements, the rotor would be stopped and each analytical cell ortube brought individually before the projectionA screen for measurementof the sedimented or packed blood-cells or other material in eachanalytical cell.

In the form of construction illustrated in Fig. 13, a rotor 18 is formedof plastic which is provided with a plurality of cell-retaining holes oropenings 19, 80, 8i, 82, to each removably receive an analytical cell39. By locating the cell-retaining holes at various radial distancesfrom the axis of the rotor, the analytical cells therein can besubjected to gravitational forces of different strengths. This is ofadvantage in ultracentriiugal runs of long duration, that is, insedimentation equilibrium runs, since it permits of simultaneouslycentrifuging several fluid samples contained in the different analyticalcells located at various distances from the center of rotation of therotor and consequently subject to centrifugal or .gravitational forcesoi different strengths. The progress of the sedimentation can beobserved visually or recorded photographically "oy training separateoptical systems on each annular lane through which the cell travelsduring rotation, or by arranging the camera in such manner that it maybe shifted into any desired radial position from the periphery down tothe center of rotation.

In the form of the invention illustrated in Figs. i4 and l5, anelliptical cell-containing rotor S3 plastic has an ellipticalcell-retaining opening all therein near the periphery and adapted tofrictionally removably receive a correspondinglyshaped plasticelliptical analytical cell Gi, although. oi course, the cell and rotorcould have the circular form, or any of the other features of theconstructions heretofore described. 'Integrally united by cementing orotherwise to one or both sides of the plastic rotor element $3, orformed therewith from one piece of plastic, are drivers ordriver-elements or turbines of plastic, the driver-elements 86 havinggenerally-circular peripheries provided with fiutings or blades designedto be engaged by one or more streams of compressed air or other fluiddirected through one or more of the four nozzles 8l, 88, 89 and at. Byhaving one or more driver-elements 8d provided with the driving-bladesor iiutings, it is possible with a given air-pressure available, tosecure a higher speed of rotation than is possible where the airdirectly drives the utings on the outer surface of the large circularrotor such as shown in Figs. l, 2, 3, 7, 8, 9, 10, 11, l2 and 13.

If it is desired to eliminate thedanger of all heat-convection currentsarising from friction o1 the rotor against the surrounding air andagainst the stream of air or other iiuid used for driving the rotor, therotor-element accommodating the analytical cell may be separated fromthe driver or turbine part, with the rotor-element rotating in avacuum-chamber in a vacuum, or an atmosphere of hydrogen at reducedpressure, for example, of twenty millimeters pressure, and the driver orturbine-elements located outside of the vacuum-chamber. Three differentways of accomplishing this result are illustrated in the three differentconstructions shown in Figs. 16, 17 and 18.

In the form of the invention illustrated in Fig.

16, the cell-containing rotor 9| and the driverss` or turbines 92 and 93are all of plastic andare integrally united together by cementing or byforming of a single piece of plastic. The ends of the steel shaft 94 aresuitably mounted in suitable -bearings (not shown), for example, like inFig. 2, and the drivers 92 and 93 can be driven by fluid under pressure,as has been previously described in connection with other forms of theinvention. The cell-containing rotor 9| is enclosed within avacuum-chamber diagrammatically indicated by the broken-line outline 95,which may be of any usual or well-known construction and may have anysuitable or usual oil-glands or other devices at the locations where theplastic hub-portions 96 of the rotor extend through the steelvacuum-chamber 95 to prevent entry of air. Windows 91 and 9B in thevacuumchamber are in line with the analytical cell 39 at one position ofrotation of the rotor 9i.

The form of invention shown in Fig. 17 is very similar to that shown inFig. 16 except that the rotor S9 is mounted upon a steel shaft l 00,which latter extends through any suitable or usual glands in thevacuum-chamber IDI indicated in broken-line outline, the axle 00 havingdrivers i02 and ID3 secured adjacent the opposite endportions of theaxle, the ends of the axle being mounted in any suitable bearing (notshown). The drivers 102 and 103 may be of plastic or any suitablematerial such as metal.

In the form of the invention illustrated in Fig. 18, the cell-containingrotor i0@ isformed of plastic and is suitably secured to a steel shaftH05 which may be of piano wire, which extends vertically through anysuitable or usual gland in a vacuum-chamber diagrammatically illustratedlby the broken line H06, and mounted in a suitable bearing l0'l, whichmay be an end-thrust R. M. B. miniature ball bearing. Secured on theupper end of the shaft 05 is a' driver or turbine lll of plastic orother suitable material for driving the rotor by a stream of uid underpressure in a way heretofore more fully described.

Removable analytical cells and balancing cells of plastic, made asdisclosed in this application can also be advantageously employed inrotors made of metal. Analytical and balancing cells as heretofore madehave consisted of assemblies of materials of di'erent densities, such asmetal barrels and screws and non-metallic spacers and windows, whichresult in different stresses set up in dierent portions of the cellduring rotation of the rotor, resulting in deformation of the metallbarrels and screws and breaking or deformation of the windows. Cellsmade according to my invention, however, are substantially unitary andhomogeneous throughout and due to bursting stress imposed on the rotorat the location of pressure of the cell in its retaining-opening is lessthan that of the higher specific gravity cells heretofore used.

'Ihe invention may be carried out in other specific ways than thoseherein set forth without 'departing from the spirit and essentialcharacteristics of the invention, and the present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended these facts and the propertiesof plastics that 7g to be embraced therein.

I claim:

l. A centrifuge construction comprising: a rotor formed substantially ofsheets of plastic integrally united together; at least one of saidsheets of plastic being formed to provide a cellchamber within andadjacent the periphery of the rotor.

2. A centrifuge construction comprising: a rotor formed substantially ofsheets of plastic integrally united together; at least one of saidsheets of plastic being'formed to provide a cellmember within andadjacent the periphery of the rotor; the sheet or sheets of plasticwhich provide the cell-chamber being of relatively-lowlight-transmitting plastic, and the sheets of plastic which overlie theopposite sides of the cellchamber being of relatively-highlight-transmitting transparent plastic.

3. In an ultracentrifuge, a rotor body consisting substantially entirelyof a plastic composition having a modulus of elasticity of the order of1,000,000, and means supporting said rotor for rotation about an axis ofsymmetry of said rotor, said rotor having at least one cavity spacedfrom said axis of symmetry and adapted to support material to besubjected to centrifugal force.

4. In an ultracentrifuge, a rotor body consisting substantially entirelyof a plastic composition having a modulus of elasticity of the order of1,000,000 and having a specic gravity of the order of one, and meanssupporting said rotor for rotation about an axis of symmetry of saidrotor, said rotor having at least one cavity spaced from said axis ofsymmetry and adapted to support material to be subjected to centrifugalforce.

5. In an ultracentrifuge, a rotor comprising a discoidal memberconsisting of a plastic composition having a modulus of elasticity ofthe order of 1,000,000, said discoidal member having at least one cavityin the body thereof between the center and the periphery thereof adaptedto support material to be subjected to centrifugal force, and meanssupporting said discoidal member for rotation about an axis of symmeterythereof.

6. In an ultracentrifuge, a rotor comprising a discoidal memberconsisting of a plastic composition having a modulus of elasticity ofthe order of 1,000,000, said discoidal member having at least one cavityin the body thereof between the center and the periphery thereof adaptedto support a container for material to be subjected to centrifugalforce, and means supporting said discoidal member for rotation about anaxis of symmetry thereof.

7. In an ultracentrifuge, a rotor comprising a discoidal memberconsisting of a plastic composi- 8. In an ultracentrifuge, a rotorcomprising a m discoidal member consisting of a plastic compositionhaving a modulus of elasticity of the order of 1,000,000, said discoidalmember having at least one cavity in the body thereof adjacent theperiphery thereof adapted to support material to be subjected tocentrifugal force, and means supporting said discoidal member forrotation about an axis of symmetry thereof.

KURT G. STERN.

